Former IRA commander in Belfast Brendan Hughes, Old Bailey bomber Dolours Price and ex-IRA leader Ivor Bell all implicated Adams during interviews they gave as part of the oral history project.

This meant that the only evidence presented by police to the PPS against Adams was hearsay evidence, which can be difficult to prove in court.

Prosecutors would have been reluctant to proceed if they felt there was a lack of clarity in relation to specific allegations made during the interviews.

They would also have had to look at the reliability of those making the allegations. In this case all three interviewees were admitted terrorists and openly hostile to those they were making allegations about.

Applications to a judge to allow hearsay evidence in a criminal case can be successful, but when it is the key evidence the court will insist on a high level of scrutiny.

In this case the PPS did not believe the hearsay evidence would stand up to such scrutiny.

The cases against Sinn Fein chairman Bobby Storey and a third man also centred on hearsay.

Although a decision was taken last year to prosecute Bell (78) for soliciting the murder of Jean McConville based on interviews he gave to Boston College, it is easier to rely on someone's own admission than their allegations against someone else.

The four females reported to the PPS were all alleged to have been present at the abduction of Mrs McConville and to have played an active role.

However, with no evidence or witnesses at the scene to identify the women, it would have been extremely difficult for the PPS to prove the allegations against them.